This invention relates generally to the field of frequency shift keying detector circuits and methods utilized for the demodulation of a data signal from a modulated carrier signal. More particularly, the present invention relates to a digital circuit implementation of a frequency shift keying detector suitable for fabrication on an integrated circuit.
In its simplest terms, a data signal, typically a digital data signal, may be used to modulate the amplitude, the frequency, or the phase of a carrier signal depending upon the particular application. Respectively, these three types of modulation are known as amplitude shift keying ("ASK"), frequency shift keying ("FSK"), and phase shift keying ("PSK"). In any of these modulation techniques, the modulated carrier signal takes on one of two states, that is, either one of two amplitudes, two frequencies, or two phases. The two states of the modulated signal then represent either a logic "zero" or a logic "one."
As noted above, changing the frequency of the modulated carrier signal to denote either of two digital logic states is called FSK modulation. FSK modulation offers a number of advantages in certain applications over other modulation techniques with respect to noise immunity and average signal power level. However, in conventional FSK detectors, it is necessary to have a reference frequency associated with the detector in order to determine whether a change in frequency denoting a data bit one or zero has occurred. If a reference frequency is available, however, detecting the data signal is normally a simple process. The input carrier signal is compared with the reference frequency. If the frequency of the carrier signal is higher than the reference frequency a data bit one may be assigned, and if the frequency of the carrier signal is lower than the reference frequency, a data bit zero may be assigned.
Therefore, while it would be advantageous to use FSK modulation techniques to transmit data to an integrated FSK detector, providing a reference frequency with a tight tolerance on an integrated circuit is extremely difficult. An absolute reference frequency with tightly controlled tolerances is difficult to achieve on an integrated circuit because of the variation in process, alignment, and other manufacturing variables. An absolute tolerance of 20% to 30% in a reference frequency provided by, for example, an on-chip oscillator is typical without resorting to off-chip precision resistors, capacitors, or crystal oscillators. Further, neither the loose tolerance of an integrated circuit implementation nor the addition of external precision components is desirable in an FSK detector. The traditional integrated circuit implementation degrades the accuracy of the FSK detector and off-chip precision components increase the cost.
What is desired is an integrated circuit implementation of an FSK detector that will accurately detect an FSK modulated carrier signal, given the limitations of integrated circuit processing, and particularly with respect to the lack of a precision on-chip reference frequency.